Why? I think we all know why. If you don't, well, go and take a look at who's playing Motoko Kusanagi - Scarlett Johansson. Casting a white actress - even one that I like - to play an Asian character immediately sank the movie in my eyes. Otherwise, it would have been no problem at all if they had cast someone who is, you know, Japanese. I'm not alone in my feelings and the choice of casting Johansson has been the singular point of criticism ever since the casting was announced.
I've seen the movie defended with arguments that Kusanagi being played by a white actress is irrelevant because of the nature of the character's ability to change bodies and well, that argument is pretty irrelevant itself because at the end of the day the character is Japanese. Changing that is just plain as day white-washing and racist. It's also nothing new. A few years back, Warner Bros was going to do a live-action adaption of Akira and if I remember correctly, the project was sunk when word got out that they wanted to cast white actors and move the location from Neo-Tokyo to New York City.
It also brings to mind The Last Airbender, the live-action adaption of Avatar: The Last Airbender that was resoundingly lambasted for its casting choices and flopped at the box office. Just like Ghost in the Shell and any movie that white-washes or otherwise miscasts characters of color should because some studio exec things that it's the way to go. It's the 21st century folks, come on.